96 INSECTS AND MAN 



marshy places, and even now the belief dies hard in the 

 minds of many people, but it has been proved beyond 

 dispute, as we shall see later, that by the bite of infected 

 mosquitoes, and by that means alone, can this dread disease 

 be transmitted from man to man. In studying the life- 

 history of the malaria mosquito we shall see that pools of 

 still water are an essential to the insect's existence, hence 

 the association of malaria with ill-drained districts. A 

 humid atmosphere, of itself, can no more introduce the 

 malaria parasite into one's system than can an evil odour 

 infect one with the germs of typhoid fever ; nor, for that 

 matter, can healthy mosquitoes, even malaria mosquitoes, 

 infect their victims with malaria, unless they have previ- 

 ously sucked the blood of a malaria patient. 



We have mentioned that malaria is not a city disease ; 

 nevertheless it does, at times, occur in certain cities. In 

 such cases it will usually be found that the town or city 

 is situated on the borders of a so-called malarial belt, that 

 is to say, a marshy district where malaria mosquitoes 

 breed. Even then, the prevalence of the disease in the 

 urban districts depends largely on the direction and force 

 of the prevailing evening breezes of the summer season; 

 depends, in short, on whether the mosquitoes are carried 

 towards the town by the wind when they become active 

 at nightfall, for all mosquitoes avoid the sunlight and heat 

 of the day as much as possible. Washington is a case in 

 point. At one time malaria, though essentially a rural 

 disease, was rife in the city; to the south lay a marshy 

 area of considerable extent, known as the Potomac Flats, 

 where malaria mosquitoes abounded. During the summer 

 the prevailing night breezes were southerly, so that here 

 were found all the essentials requisite for the spread of the 

 disease in the city. The mosquitoes on the Flats, hungry 

 for human blood, and rendered energetic by the relative 

 coolness of the evening air, were gently wafted towards 

 the sleeping people of Washington. Some, perchance, took 



